云南白水台钙华景区的水化学和碳氧同位素特征及其在古环境重建研究中的意义

HYDROCHEMICAL AND ISOTOPIC CHARACTERISTICS OF TUFA IN THE BAISHUITAI SCENIC AREA OF YUNNAN AND THEIR IMPLICATIONS FOR PALEOENVIRONMENT RECONSTRUCTION

采用水化学仪器自动记录、现场滴定和样品碳氧稳定同位素测试相结合等方法 ,对云南中甸白水台钙华景区的水化学和碳氧稳定同位素特征进行了综合分析。主要结论是 :形成白水台钙华的泉水具有很高的钙和重碳酸根离子浓度 ,相应地 ,泉水的CO2 分压显著高于土壤生物成因所能产生的CO2 分压。结合泉水出露的地质条件及其碳稳定同位素特征 (δ1 3C=- 1 .2 3‰ )的分析 ,进一步发现 ,高CO2 分压主要与深部地热成因的CO2 有关 ,而非原来普遍认为的“是温暖湿润气候的产物”。可见 ,白水台钙华属于热成因类钙华。由此 ,根据白水台不同时代钙华氧稳定同位素组成的差异 ,对钙华形成时的水温进行了计算。结果发现自白水台钙华形成以来 ,水温变化高达 1 1℃ ,即从最老 (<3 5万年 )钙华形成时的 2 1℃降至现在的 1 0℃。这可能与本地区强烈抬升导致的气温降低有关 ,也可能反映出地热对水温的影响在降低。此外 ,本文对用热成因类钙华进行古环境重建研究中值得注意的问题也做了讨论。这些问题包括放射性碳测年中“死碳”(来自深部碳酸盐碳和深部CO2 )的干扰及由深源CO2和CO2 自水中逸出导致的钙华1 3C富集 ,后者在利用类似热成因碳酸盐沉积的δ1

Methods of automatic hydrochemical logging and in-situ titrating combinied with the measurement of stable carbon and oxygen isotope were used to study the hydrochemical and isotopic characteristics of tufa in the Baishuitai scenic area, Yunnan, China. It was found that the tufa-forming springs have a very high concentration of calcium and bicarbonate, and accordingly a very high CO 2 partial pressure, which is not likely to be produced by biological activity in soil alone. Further analysis on the stable carbon isotopic composition of the spring water ( δ 13C=-1.23‰) shows that the high pressure of CO 2 is mainly related to a deep-seated CO 2 source, meaning that the Baishuitai tufa should be thermo-genetic. This is contrast to the commonly accepted saying that the tufa deposition simply is a product of warm and humid climate in the karst ecological environment. According to relevant calculations based on differences in stable oxygen isotopic compositions of the tufa depositions formed in different epochs at Baishuitai, the change in water temperature is as high as 11℃, i.e., from 21℃ at start of the Baishuitai tufa deposition (<350,000 yr), to 10℃ at present, which may reflect an effect that the strong uplift of the area, and/or the deep geothermal source weakening would have on water temperature. Problems related to paleoenvironmental reconstruction by thermogenetic tufa are also discussed. These problems include the interference of 'dead carbon' in radiocarbon dating and the 13C enrichment in tufa by deep-seated CO 2 and degassing of CO 2 from water. The latter must be considered when reconstructing the paleovegetation by using δ 13C data of thermogenetic carbonate deposits.